http://ambientmusicguide.com/ You all should check out Ambient Music Guide by Mike G. His website has a ton of information on Ambient artists and their best works plus he also produces his own mixes which are wonderful to listen to.
As per myself my favorite ambient music ever is Richard D. James aka Aphex Twin
All of these are from his second album under Aphex Twin titled "Selected Ambient Works Volume II". It is a double disc ambient album that is my favorite ambient album to date. The whole album is amazing from beginning to end and has arguably his greatest works of art in that of Stone In Focus and Rhubarb. I love him and own everything he has produced.
Outside of him I'm a big fan of Global Communication, Biosphere, Tetsu Inoue, Robert Rich, Lustmord, Steve Roach, Pete Namlook (RIP), Brian Eno and a whole slew of others.
I love the ticking clock throughout the song. 76:14 by Global Communication is a gem amongst the Ambient landscape. Released in 1994 this album is unique in that each song title is the length of the track and the album title is the total running time of the album
Another wonderful track but this time by Norwegian producer Geir Jenssen better known as Biosphere. This is taken off of his landmark 1997 release Substrata. This album is what mother nature would sound like if she could produce her own music.